Analysis of the Sulfur Cathode Capacity Fading Mechanism and Review of the Latest Development for Li-S Battery

Because of automotive industry sustainable development, the demand for higher energy density rechargeable batteries make the lithium-sulfur (Li-S) batteries become one of the most attractive candidates.The Li-S systems have a theoretical specific energy of 2600 Wh/kg while the theoretical capacity of sulfur is 1680 mAh/g.Sulfur is abundant, low cost and environment friendly.Although the rechargeable Li-S batteries possess more advantages over the conventional lithium ion batteries, the practical use faces with a variety of problems such as low specific capacity and short cycle life.Based on the fundamental electrochemical process of the sulfur cathode, the capacity fading mechanism of the sulfur cathode is analyzed in details.Combining with the works of our research team, the factors leading to the cathode property fading mechanism are summarized.Firstly, the main issue is that sulfur is both ionically and electrically insulating.And the insoluble low-order lithium polysulfide discharge products are also expected to be electronic insulators.So the cathode structure must contain electronic conductors (carbon or metal powder) which will decrease the energy density.Secondly, researchers impute the capacity fading into the residual Li 2 S 2 and Li 2 S in sulfur cathode even at 100% depth of charge.The formation of Li 2 S 2 and Li 2 S increasing with cycling results in active material loss.And the deposition of irreversible Li 2 S or Li 2 S 2 at cracked surfaces of carbon particles causes cathode structural failure.Thirdly, high ordered lithium polysulfide (Li 2 S n , 3≤n≤8) is soluble in electrolyte, but low ordered lithium polysulfide (Li 2 S 2 and Li 2 S) is insoluble.Thus chemical precipitation/dissolution reactions occur during the electrochemical process resulting in active material transition between liquid phase and solid phase.But it is difficult for the high ordered lithium polysulfide to transfer completely from liquid phase to solid phase at the end of cycles, so that will lead to the active material loss.Fourthly, another serious problem is the irreversible oxidation of cathode active material.The formation of Li x SOy species increasing with cycling indicates an important capacity fading mechanism of Li-S battery.In this paper, the main research directions and the latest development to enhance the performance of sulfur cathode are reviewed from the aspects of carbon conductive structure, polymer coatings and metal oxides additives, and also the problems in each research directions are analyzed.Finally, the further development of Li-S battery is discussed.